CASSCF and CASPT2 Ab Initio Electronic Structure Calculations Find Singlet Methylnitrene Is an Energy Minimum

Abstract

(12/11)CASSCF and (12/11)CASPT2 ab initio electronic structure calculations with both the cc-pVDZ and cc-pVTZ basis sets find that there is a barrier to the very exothermic hydrogen shift that converts singlet methylnitrene, CH 3 N, to methyleneimine, H 2 C NH. These two energy minima are connected by a transition structure of C s symmetry, which is computed to lie 3.8 kcal/mol above the reactant at the (12/11)CASPT2/cc-pVTZ//(12/11)CASSCF/cc-pVTZ level of theory. The (12/11)CASSCF/cc-pVTZ value for the lowest frequency vibration in the transition structure is 854 cm -1, and CASPT2 calculations concur that this a‘ ‘ vibration does indeed have a positive force constant. Thus, there is no evidence that this geometry is actually a mountain top, rather than a transition structure, on the global potential energy surface or that a C 1 pathway of lower energy connects the reactant to the product. Therefore, our computational results indicate that the bands seen for singlet methylnitrene in the negative ion photoelectron spectrum of CH 3 N - are due to singlet methylnitrene being an energy minimum, rather than a transition state. Our results also lead us to predict that, at least in principle, singlet methylnitrene should be an observable intermediate in the formation of methyleneimine.